The invention relates to a magnetic disk apparatus for reading medium information by an MR head and, more particularly, to a magnetic disk apparatus which can properly recover in the case where a sync byte pattern at the head of a sector is lost by a thermal asperity in which a read signal suddenly fluctuates by a collision heat generation when an MR head comes into contact with a medium.
In recent years, in association with an increase in capacity of a magnetic disk apparatus as an external storage device of a computer, a magnetic head of a high performance is needed. As a magnetic head satisfying the need, attention is paid to a magnetic disk apparatus having what is called an MR head using a magneto-resistive element which can obtain a high reproduction output without depending on a peripheral velocity of a recording medium. In the magnetic disk apparatus using such an MR head as a read head, however, when the MR head collides with a physical convex or concave portion due to an extremely slight dent, a deformation, or the like on the surface of the medium which is rotating, a temperature of the MR head rises instantaneously by a friction heat. When the temperature of the MR head rapidly rises by the contact with the thermal fluctuation concave and convex portions of the medium as mentioned above, a base line of a read signal is shifted, and a read error which cannot be recovered occurs. This state is seemingly the same as that a medium defect exists. The phenomenon in which the read error occurs by the collision heat generation of the MR head with the medium is usually called a thermal asperity. That is, when a state in which a sync byte pattern cannot be read by the thermal asperity of the MR head occurs in a sync byte region in a read sector on a medium track, sector data cannot be demodulated at all. In this case, although the reading operation is executed again, since the thermal fluctuation concave and convex portions are kind of physical defects, the same reading impossible state repetitively occurs in the same sync byte region and an unrecoverable read error is caused. For a high density recording of the medium, it is necessary to reduce a floating height of the MR head and this results in a factor of an increase in number of times of occurrence of the defect due to the thermal asperity of the MR head. As a rotational speed of the medium increases, when the thermal asperity of the MR head occurs, the shift of the base line of the read signal further increases. Further, every possible is performed on the magnetic disk apparatus at the factory. The defect caused by the thermal asperity of the MR head is, however, a problem occurring during the use by the user. Further, since there is a tendency such that the defect grows while the user is using the apparatus, there is a fear that the performance of the magnetic disk apparatus remarkably deteriorates.